Enhanced Submodule Selection Algorithm for Mitigating FB-SM Overvoltage and HB-SM Undervoltage in Hybrid MMCs During DC Faults

Blocking all submodules (SMs) of hybrid modular multilevel converters (HMMCs) is a simple and rapid way to clear dc fault currents. However, full-bridge submodules (FB-SMs) must absorb nearly all of the fault energy stored in inductors and dc transmission lines, which can potentially cause significant FB-SM overvoltage when the transmission line is excessively long or the FB-SM ratio is low. In addition, half-bridge submodules (HB-SMs) are prone to undervoltage due to over-discharging before SM blocking, which hinders the HMMC’s restart. Current solutions, such as increasing SM capacitance and adding more FB-SMs, are not cost-effective. This article addresses the challenge of clearing dc fault currents in HMMCs by proposing an enhanced SM selection algorithm. Notably, the proposed method requires no additional hardware investment, does not disrupt normal system operation, and is compatible with existing FB-SM overvoltage mitigation techniques.